In a conventional plant for synthesizing methanol from a hydrocarbon gas or synthesizing gasoline from a hydrocarbon gas via methanol, there is the problem that uses for waste heat of not greater than approximately 150° C. are limited, and a lot of heat is discarded. In plants for synthesizing methanol or gasoline, measures are taken in which excess steam is introduced when steam-reforming a hydrocarbon gas such as natural gas to prevent carbon deposition during steam reforming. For this reason, a large amount of steam, in addition to hydrogen and carbon monoxide which are primary components of the reformed gas obtained by the steam reforming, remain in the reformed gas. While the reformed gas has high heat of condensation, a lot of the heat is discarded without being used because a lot of steam condenses at temperatures not greater than 150° C.
Japanese Unexamined Patent Application Publication No. 2003-34503A discloses that waste heat is recovered from a reformed gas using a plurality of heat exchangers and effectively used as a heat source of distillation columns that distill methanol. Japanese Unexamined Patent Application Publication No. 2006-213580A discloses that reboilers are configured in multiple stages in a regeneration column of a carbon dioxide absorption liquid, and as the heat source of these multi-stage reboilers, a plurality of steam of different pressures extracted from a turbine is used.